Bowling Ball Mortar Question

[agent141]

Ok , built the BB Mortar (can't post a pic due to rules of this forum) the oxy. cyl. has a separate powder chamber welded to it. The monolithic chamber measures 1.5"  dia x 3" deep has a conical bottom and a concave top (bb radius) fuse hole is 1.5" from the bottom, it has a 1.5" wall thickness more at the bottom and more at the top due to the weld and oxy. tank neck thickness. The chamber was welded inside and out (no small feat). My question is for the others who have built bb mortars w/ separate powder chambers, I am still leery of this thing (hiding behind the truck when I set it off) I am running what I consider light loads - 16 lb ball - 650gr Cannon grade powder at a 70 degree angle I am getting 120yd avg for the distance and who knows what altitude. What are you guys loading? with powder charge, ball and angle. what is still a safe load?. am I close?. guess I am just overly cautious and don't want to make a bomb. Any replies appreciated.

[Double D]

If I interpret what you have posted correctly you are saying the  Powder chamber is  4.5 inches in outside diameter and 1.5 inches in inside diameter.  Then it is 3 inches deep. 

I would say the powder chamber is really to small for this gun.  1.5 inch diameter chamber is at the top end of the small cannon and  just below the big cannon scale.

Switlik's chart shows Maximum charge for 1.5 inch as 500 grains. Doesn't say which grade of powder to use.   

I would say if you are using 650 grains of  Cannon you probably okay. 

I am concerned about one other thing how ever.  If you are so afraid of this cannon that you run and hide at every shoot why are you even firing it.  Caution is good, fear s not.

[GGaskill]

...16 lb ball - 650gr Cannon grade powder at a 70 degree angle...

You're using about 1.5 oz of powder; the people I know who have BB mortars use anywhere from 4 to 8 oz of cannon grade, although their mortars are made from thick wall cylinders and do not have a separate powder chamber.  If you are looking for longer range, you shouldn't use as steep an elevation; get closer to 45°. 

[agent141]

Double D, not really running and hiding just overly cautious.... I also look 3 or4 times before I cross the street . I would like to be confident enough with my set up to have spectators and I within 20 -25 feet behind when I touch it off... Back to the chamber I really thought I made it too big when I was designing it, at 650gr it is still only one third full... GGaskill. not really looking for more range I know I can always lower the angle I just think it is neat as hell to see a BB go up almost out of sight.   thanks for the replies guys keep-um cumming.

[cannonmn]

Personally I'd hesitate to comment unless I examined the item in person, but then I'm no metallurgical expert either.  One issue that bowling ball mortar makers need to keep in mind is that commercial gas cylinders are manufactured and tested in a certain way for holding compressed gases under certain conditions.  The folks who use them more or less "as is" and only cut down are taking advantage of the original features, materials, heat treatment, etc.  

I would be a bit concerned that welding on these cylinders without thinking through the details of what the welding heat does to the original heat treatment, could weaken the cylinder.  Maybe these cylinders, even if unevenly annealed, are strong enough for BB mortar use when using small powder charges, but I don't know.  Just keep in mind that the more you change it from the way it was made, the more unknowns you introduce.

Since the use of BB mortars seems to be increasing, I'd like to see a competent engineer go through the math of gas cylinder design specs vs. their use as BB launchers.  That means to me using the hoop-strength formulas that are typically used for gun barrel design, and applying them to commercial gas cylinders.  The immediate objective would be to analyze a typical gas cylinder and determine, by the hoop strength formulas, what the maximum pressure is that they can withstand, using typical safety factors of say 2:1 or more.  Then I'd like to see what internal pressures we're dealing with, given various types and weights of black powder.

I've been out of school too long to feel competent to do this, but I'd really like to see someone go through the numbers here.

[Double D]

John,

The heat treat of the cylinder is not really relevant as the cylinder is no longer a pressure vessel.   In Gas cylinder bowling ball mortars, the gas cylinder portion is little more than an expansion chamber and ball holder. The benefit of the cylinder isn't derived from it's construction and strength but its shape.   The  powder chamber is the cannon.  That has to be built to the standards.  That doesn't mean care shouldn't be taken with design and construction. 

[cannonmn]

>The cannon is the powder chamber.

Hmmm, I guess I'm lost here, need more info.  I've only seen a few BB mortars but the way they are designed, the powder is dumped into what was the top of the gas cylinder, and the bowling ball rolled in on top of the powder.  If there's another way it is done that's not the way I was referring to.  So in the ones I've actually seen the top of the gas cylinder is the chamber and the barrel both.

Maybe there's some guide to building BB mortars that has a separate chamber specified, I don't know, if so where do I find out about that kind?

[Double D]

Transposed words  John.   the Powder Chamber is the cannon.  I have not seen one that has the powder just dumped in the bottom of the tank.  At the minimum the powder was poured in the neck and that is the chamber.

Now having said that, I don't doubt your scenario one bit. After reading some of things people do with out thinking, it would not  be a surprise to me if someone did just that.  We can't do much about fools.

If you follow the safety rules and guidlines laid out by  N-SSA  and AAA for the safe firing of cannons and mortars you will see you need to have   one caliber wall  thickenss around the chamber.  So if you dump youy charge in the bottom of the cylinder and drop the ball in on top you need to hav walls the  are 8.6 inches thick.  Total barrel diameter  almost  26 inches in diameter.    That's not practical.  In order to properly build the mortar  and use the tank as the profile and make it safe you have to have a chamber. 

[Artilleryman]

a 70 degree angle I am getting 120yd avg for the distance and who knows what altitude.

I just think it is neat as hell to see a BB go up almost out of sight.   thanks for the replies guys keep-um cumming.

You may want to lower the angle to less than 70 degrees, or at least do not shoot in to a strong wind.  With a the height that you are apparently getting the ball may drift an come down a little closer to you than you might be comfortable with.  I am reminded of an old adage about not spitting into the wind or something to that effect.    The N-SSA limits the angle of fire to no less than 30 and no more than 60 degrees.  I am not aware of why that rule was put into effect other than a safety measure.  I have watched light weight projectiles drift quite a bit in a strong cross wind.

Having shot eight inch siege mortar to a range of 900-1000 yards and watched the projectile almost disappear from sight it would appear that you have a large enough powder charge.

I am glad that you joined the forum and have asked questions. 

[agent141]

cummon guys....  anybody with a BB mortar share your loading data / results.....

[12741]

My BB mortar has a 2"x2" powder chamber and will throw a ball 400+ yards down range at 45 deg.
with 3 OZ. cannon grade Goex. I now use 2FA ,fireworks grade powder and get same results.
You can buy it for $4 pound in 25 Pd. bags.

[Cat Whisperer]

My BB mortar has a 2"x2" powder chamber and will throw a ball 400+ yards down range at 45 deg.
with 3 OZ. cannon grade Goex. I now use 2FA ,fireworks grade powder and get same results.
You can buy it for $4 pound in 25 Pd. bags.

Good price!  Source?

[12741]

I have friends who are members of pryrotechnic guild,I will ask if they will provide that information.
I havent been too inquisitive,they offered the powder as a favor and I accepted.

[Double D]

Check with the powder distributors on the cannon supplies list they have 1FA. 1 FA is sold in 25 lb Polyethylene Bags.  But also note there are some restrictions on its sale. Powder Inc. has this warning Polyethylene Bags are only available with commercial ATF license.